Dans un premier temps il faut que Composer soit installé sur votre système.

Ensuite sous LINUX : rendez-vous dans le répértoire contenant votre composer.phar et executez:

php composer.phar create-project symfony/website-skeleton mon-projet

Là composer installe le projet dans le répértoire mon-projet.

Sous WINDOWS : Rendez vous dans le répértoire de base qui contiendra le répertoire de travail de votre projet et tapez :

composer create-project symfony/website-skeleton nom-du-repertoire-du-projet

nom-du-repertoire-du-projet est le nom du répertoire qui va etre créé par Composer dans le répertoire courant.

Si il donne des erreurs, réglez les. C'est souvent un manque d'extension PHP.

Résultat:

D:\webprojects>composer create-project symfony/website-skeleton test-api-project
Installing symfony/website-skeleton (v4.3.99)
  - Installing symfony/website-skeleton (v4.3.99): Downloading (100%)
Created project in test-api-project
Loading composer repositories with package information
Updating dependencies (including require-dev)
Package operations: 1 install, 0 updates, 0 removals
  - Installing symfony/flex (v1.4.6): Downloading (100%)
Symfony operations: 1 recipe (0216ef10f1722e46db96c3cadf9b7a02)
  - Configuring symfony/flex (>=1.0): From github.com/symfony/recipes:master
Loading composer repositories with package information
Updating dependencies (including require-dev)
Restricting packages listed in "symfony/symfony" to "4.3.*"
 
Prefetching 103 packages 
  - Downloading (100%)
 
Package operations: 103 installs, 0 updates, 0 removals
  - Installing ocramius/package-versions (1.5.1): Loading from cache
  - Installing doctrine/lexer (1.1.0): Loading from cache
  - Installing doctrine/annotations (v1.7.0): Loading from cache
  - Installing doctrine/reflection (v1.0.0): Loading from cache
  - Installing doctrine/event-manager (v1.0.0): Loading from cache
  - Installing doctrine/collections (v1.6.2): Loading from cache
  - Installing doctrine/cache (v1.8.0): Loading from cache
  - Installing doctrine/persistence (1.1.1): Loading from cache
  - Installing symfony/polyfill-php73 (v1.12.0): Loading from cache
  - Installing symfony/polyfill-mbstring (v1.12.0): Loading from cache
  - Installing symfony/polyfill-php72 (v1.12.0): Loading from cache
  - Installing symfony/polyfill-intl-idn (v1.12.0): Loading from cache
  - Installing symfony/mime (v4.3.4): Loading from cache
  - Installing symfony/http-foundation (v4.3.4): Loading from cache
  - Installing symfony/event-dispatcher-contracts (v1.1.5): Loading from cache
  - Installing psr/container (1.0.0): Loading from cache
  - Installing psr/cache (1.0.1): Loading from cache
  - Installing symfony/event-dispatcher (v4.3.4): Loading from cache
  - Installing psr/log (1.1.0): Loading from cache
  - Installing symfony/debug (v4.3.4): Loading from cache
  - Installing symfony/http-kernel (v4.3.4): Loading from cache
  - Installing symfony/service-contracts (v1.1.6): Loading from cache
  - Installing symfony/dependency-injection (v4.3.4): Loading from cache
  - Installing symfony/routing (v4.3.4): Loading from cache
  - Installing symfony/finder (v4.3.4): Loading from cache
  - Installing symfony/filesystem (v4.3.4): Loading from cache
  - Installing symfony/config (v4.3.4): Loading from cache
  - Installing symfony/var-exporter (v4.3.4): Loading from cache
  - Installing symfony/cache-contracts (v1.1.5): Loading from cache
  - Installing symfony/cache (v4.3.4): Loading from cache
  - Installing symfony/framework-bundle (v4.3.4): Loading from cache
  - Installing sensio/framework-extra-bundle (v5.4.1): Loading from cache
  - Installing symfony/asset (v4.3.4): Loading from cache
  - Installing symfony/dotenv (v4.3.4): Loading from cache
  - Installing symfony/expression-language (v4.3.4): Loading from cache
  - Installing symfony/inflector (v4.3.4): Loading from cache
  - Installing symfony/property-access (v4.3.4): Loading from cache
  - Installing symfony/options-resolver (v4.3.4): Loading from cache
  - Installing symfony/intl (v4.3.4): Loading from cache
  - Installing symfony/polyfill-intl-icu (v1.12.0): Loading from cache
  - Installing symfony/form (v4.3.4): Loading from cache
  - Installing symfony/http-client-contracts (v1.1.6): Loading from cache
  - Installing symfony/http-client (v4.3.4): Loading from cache
  - Installing symfony/stopwatch (v4.3.4): Loading from cache
  - Installing symfony/console (v4.3.4): Loading from cache
  - Installing zendframework/zend-eventmanager (3.2.1): Loading from cache
  - Installing zendframework/zend-code (3.3.2): Loading from cache
  - Installing ocramius/proxy-manager (2.2.3): Loading from cache
  - Installing doctrine/dbal (v2.9.2): Loading from cache
  - Installing doctrine/inflector (v1.3.0): Loading from cache
  - Installing doctrine/common (v2.11.0): Loading from cache
  - Installing doctrine/migrations (2.1.1): Loading from cache
  - Installing symfony/doctrine-bridge (v4.3.4): Loading from cache
  - Installing doctrine/doctrine-cache-bundle (1.3.5): Loading from cache
  - Installing symfony/security-core (v4.3.4): Loading from cache
  - Installing jdorn/sql-formatter (v1.2.17): Loading from cache
  - Installing doctrine/doctrine-bundle (1.11.2): Loading from cache
  - Installing doctrine/doctrine-migrations-bundle (v2.0.0): Loading from cache
  - Installing symfony/yaml (v4.3.4): Loading from cache
  - Installing doctrine/instantiator (1.2.0): Loading from cache
  - Installing doctrine/orm (v2.6.4): Loading from cache
  - Installing symfony/orm-pack (v1.0.6): Loading from cache
  - Installing symfony/security-http (v4.3.4): Loading from cache
  - Installing symfony/security-guard (v4.3.4): Loading from cache
  - Installing symfony/security-csrf (v4.3.4): Loading from cache
  - Installing symfony/security-bundle (v4.3.4): Loading from cache
  - Installing symfony/serializer (v4.3.4): Loading from cache
  - Installing symfony/property-info (v4.3.4): Loading from cache
  - Installing webmozart/assert (1.5.0): Loading from cache
  - Installing phpdocumentor/reflection-common (2.0.0): Loading from cache
  - Installing phpdocumentor/type-resolver (1.0.1): Loading from cache
  - Installing phpdocumentor/reflection-docblock (4.3.2): Loading from cache
  - Installing symfony/serializer-pack (v1.0.2): Loading from cache
  - Installing egulias/email-validator (2.1.11): Loading from cache
  - Installing swiftmailer/swiftmailer (v6.2.1): Loading from cache
  - Installing symfony/swiftmailer-bundle (v3.2.8): Loading from cache
  - Installing symfony/translation-contracts (v1.1.6): Loading from cache
  - Installing symfony/translation (v4.3.4): Loading from cache
  - Installing symfony/validator (v4.3.4): Loading from cache
  - Installing psr/link (1.0.0): Loading from cache
  - Installing fig/link-util (1.0.0): Loading from cache
  - Installing symfony/web-link (v4.3.4): Loading from cache
  - Installing symfony/var-dumper (v4.3.4): Loading from cache
  - Installing twig/twig (v2.11.3): Loading from cache
  - Installing symfony/twig-bridge (v4.3.4): Loading from cache
  - Installing symfony/twig-bundle (v4.3.4): Loading from cache
  - Installing symfony/web-profiler-bundle (v4.3.4): Loading from cache
  - Installing symfony/profiler-pack (v1.0.4): Loading from cache
  - Installing monolog/monolog (1.25.1): Loading from cache
  - Installing easycorp/easy-log-handler (v1.0.7): Loading from cache
  - Installing symfony/monolog-bridge (v4.3.4): Loading from cache
  - Installing symfony/monolog-bundle (v3.4.0): Loading from cache
  - Installing symfony/debug-bundle (v4.3.4): Loading from cache
  - Installing symfony/debug-pack (v1.0.7): Loading from cache
  - Installing nikic/php-parser (v4.2.4): Loading from cache
  - Installing symfony/maker-bundle (v1.13.0): Loading from cache
  - Installing symfony/phpunit-bridge (v4.3.4): Loading from cache
  - Installing symfony/css-selector (v4.3.4): Loading from cache
  - Installing symfony/dom-crawler (v4.3.4): Loading from cache
  - Installing symfony/browser-kit (v4.3.4): Loading from cache
  - Installing symfony/test-pack (v1.0.6): Loading from cache
  - Installing symfony/process (v4.3.4): Loading from cache
  - Installing symfony/web-server-bundle (v4.3.4): Loading from cache
Writing lock file
Generating autoload files
ocramius/package-versions: Generating version class...
ocramius/package-versions: ...done generating version class
Symfony operations: 20 recipes (0216ef10f1722e46db96c3cadf9b7a02)
  - Configuring symfony/framework-bundle (>=4.2): From github.com/symfony/recipes:master
  - Configuring doctrine/annotations (>=1.0): From github.com/symfony/recipes:master
  - Configuring symfony/routing (>=4.2): From github.com/symfony/recipes:master
  - Configuring sensio/framework-extra-bundle (>=5.2): From github.com/symfony/recipes:master
  - Configuring symfony/console (>=3.3): From github.com/symfony/recipes:master
  - Configuring doctrine/doctrine-cache-bundle (>=1.3.5): From auto-generated recipe
  - Configuring doctrine/doctrine-bundle (>=1.6): From github.com/symfony/recipes:master
  - Configuring doctrine/doctrine-migrations-bundle (>=1.2): From github.com/symfony/recipes:master
  - Configuring symfony/security-bundle (>=3.3): From github.com/symfony/recipes:master
  - Configuring symfony/swiftmailer-bundle (>=2.5): From github.com/symfony/recipes:master
  - Configuring symfony/translation (>=3.3): From github.com/symfony/recipes:master
  - Configuring symfony/validator (>=4.3): From github.com/symfony/recipes:master
  - Configuring symfony/twig-bundle (>=3.3): From github.com/symfony/recipes:master
  - Configuring symfony/web-profiler-bundle (>=3.3): From github.com/symfony/recipes:master
  - Configuring easycorp/easy-log-handler (>=1.0): From github.com/symfony/recipes:master
  - Configuring symfony/monolog-bundle (>=3.3): From github.com/symfony/recipes:master
  - Configuring symfony/debug-bundle (>=4.1): From github.com/symfony/recipes:master
  - Configuring symfony/maker-bundle (>=1.0): From github.com/symfony/recipes:master
  - Configuring symfony/phpunit-bridge (>=4.3): From github.com/symfony/recipes:master
  - Configuring symfony/web-server-bundle (>=3.3): From github.com/symfony/recipes:master
Executing script cache:clear [OK]
Executing script assets:install public [OK]
 
Some files may have been created or updated to configure your new packages.
Please review, edit and commit them: these files are yours.
 
Some files may have been created or updated to configure your new packages.
Please review, edit and commit them: these files are yours.
 
 
 What's next? 
 
 
  * Run your application:
    1. Go to the project directory
    2. Create your code repository with the git init command
    3. Download the Symfony CLI at https://symfony.com/download to install a development web server
 
  * Read the documentation at https://symfony.com/doc
 
 
 Database Configuration 
 
 
  * Modify your DATABASE_URL config in .env
 
  * Configure the driver (mysql) and
    server_version (5.7) in config/packages/doctrine.yaml
 
 
 How to test? 
 
 
  * Write test cases in the tests/ folder
  * Run php bin/phpunit

Dans votre navigateur ouvrir la page http://%VotreServeurWeb%/html/truc/machin/public/ et vous devriez voir cela:

Maintenant pour naviguer dans votre futur site utilisez l'URL http://%VotreServeurWeb%/html/truc/machin/public/index.php/MaRoute

La partie MaRoute de l'URL sera votre route dans le routing de Symfony